1. Field of the Invention
The present invention generally relates to a D.C. power supply system and, more particularly, to two or more D.C. power supplies which are connected to a single load to insure continuity of power to the load in the event one of the power supplies becomes inoperative.
2. Description of the Prior Art
While it is well known that multiple power supplies within a system may be connected together in parallel to increase the total output current of the power supply system in the event a single power supply would be incapable of supplying the requisite load current for a system in which it is incorporated, such parallel connected power supplies are typically not constructed so that an individual power supply may be removed without interrupting the power supplied to the load. Additionally, a system in which multiple parallel connected supplies are incorporated may require operation of all of the individual units to supply the necessary load current for the system, and if one of the power supplies becomes inoperative due to component failure or the like, the total load current may be inadequate. However, if the remaining power supplies are marginally capable of providing the required load current for the system, the increased load on the remaining supplies may result in overheating or failure of these remaining power supplies.
In an overall system which requires a continuous supply of power because of the importance of continued service, as, for example, in portions of a telephone system, or where the equipment being supplied is of such high cost that complete redundancy of the entire equipment is economically feasible, the use of redundant power supplies may be quite practical. However, in the event two or more power supplies are connected to a common load and one of the power supplies becomes inoperative, the advantage of having the second power supply would be severely diminished if the overall system was required to be shut down while the inoperative power supply was disconnected and replaced or repaired. This problem may be overcome by the use of the OR-ing or coupling diodes.
Diode OR-ing of redundant low voltage regulated converters to a common logic load presents a different, more serious problem. The forward voltage drop of a silicon rectifier is approximately 0.8 volts, or 6.67 percent for a 12 volt supply. Further, the rectifier forward voltage drop is a function of rectifier current. The use of hot carrier (Schottky) rectifiers will result in lower voltage drops, somewhat alleviating the problem.
In order to overcome the regulation problem of the rectifier drops, remote sensing could be used. Here each regulated power supply monitors the voltage across the sense point load and the bus would be accurately maintained at the proper voltages. One drawback is evident; although both regulated power converters might be initially set to the same exact voltage, one supply may eventually drift to a voltage slightly lower than the remaining supply. Under such a condition, the lower voltage regulated supply would essentially shut down, and its output voltage, on the converter side of the OR-ing rectifier would drop to a small percentage of the required output.